Pickling of steel



Patented July 15, 1941 2,249,383 PICKLING OF STEEL Donald Cleve Hilty, Vandergrift, Pa. No Drawing. Original application December 29,

1937, Serial No.

182,321, now Patent No.'

2,178,791, dated November 7, 1939. Divided and this application August 31, 1939, Serial No.

a 2 Claims.

This invention is a division from my copending application of the same title, bearing Se-- rial No. 182,321, filed December 29, 1937, now Patent No. 2,178,791, issued Nov. 7, 1939, and re? lates to the pickling of steel and is particularly concerned with the removal of red oxide from silicon steel. In explanation, silicon steel sheets, upon being annealed, develop a reddish surface color in addition to the usual scale to be expected. Ordinary pickling does not remove this red color.

According to the present invention, a pickling bath is prepared which contains, in addition to the usual concentration of sulphuric acid, sulphur dioxide to the extent of at least .025% by weight of the solution. In some instances hyoxide, with the solution at a temperature not exceeding 140 Fahrenheit. The scale removing action of the solution may be accelerated by the addition of a small amount of hydrochloric 5 acid. The sulphur dioxide concentration may dium sulphite, sodium bi-sulphite, or sodium lease this gas upon reaction with the pickling solution. Examples of such compounds are sothiosulphate.

In the case bleaching alone is desired the A pickling solution used should not contain more than 1.0% by weight of sulphuric acid. No hydrochloric acid should be used. Immersion of drochloric acid may be added to accelerate the scale removal. Other pickling additions may be made, such as that of the various inhibitors which have become common. In case it is desired to remove the reddish color without also removing the scale, and acid concentration may be kept to a very low value, bleaching occurring if the sulphur dioxide concentration is above the factor mentioned.

out materially afiecting the normal scale. Pref- ,erably, 'a suitable inhibitor should be added to the solution.

I claim:

l. A process for removing the red color from heat treated silicon steel while effecting substan- The objectionable reddish tinge is removed from annealed silicon sheets upon immersion in either of the above solutions. In the case of a 1 solution having a normal acid concentration common to pickling liquors, both the reddish color and ordinary scale will be removed. In the case of certain types of silicon sheets the scale remains, and in such instances the solution containing a low concentration of acid but including the sulphur dioxide functions to bleach the reddish color to a uniform grayish tintf It has been found that there is an abrupt decrease in the solubility of sulphur dioxidegas in acid pickling solution as the temperature of the solution is raised above 140 Fahrenheit.

As a specific example, the removal of both ordinary scale and red oxide from annealed silicon steel sheets may be effected by immersing the sheets for ten minutes in an ordinary 7% sulphuric acid pickling solution containing, however, .025% or more by weight of sulphur disulphur dioxide being maintained at a concentration suflicient to effect removal of the red color and the acidity ofthe solution being sufficiently high to effect substantial scale' removal.

2. A process for removing the-red color from heat treated silicon steel while effecting substantial scale removal, including subjecting the steel to the action of an acid solution containing sulphur dioxide while maintaining the solution at temperatures not exceeding Fahrenheit, the sulphur dioxide being maintained at a concentration in excess of .025% by weight of the solution to eii'ect removal of the red color and. the' acidity of the solution being sufliciently high'to eflect substantial scale removal.

DONALD CLEVE HILTY. 

